1. Field of the Invention
The invention relates to titanium dioxide fine particles that can be favorably used for visible light activatable photocatalysts, semiconductors and reflection films for optical communication and a method for producing the fine particles, and a method for producing a visible light activatable photocatalyst. Particularly, the invention relates to titanium dioxide fine particles that are activated by irradiating a visible light and the titanium dioxide fine particles of the invention can be favorably used for paints, textiles, sick house syndrome alleviating agents, harmless treatment agents for industrial effluent and exhaust, and medical materials because of exhibiting functions such as removal of harmful substances by decomposition, deodorization, antibacterial action, decontamination action and anti-clouding action by taking advantage of such photocatalytic activity, and method for producing the same, and method for producing visible light activatable photocatalyst.
2. Description of the Related Art
Electrons and positive holes are formed by photoactivation when a light having an energy exceeding band gaps of a semiconductor is irradiated to semiconductor particles such as titanium dioxide. The electrons and positive holes are transferred to the surface of the semiconductor particles, and evokes various reactions called as photocatalytic reactions by interacting with neighboring ionic species and molecular species.
Since the positive holes generated on the surface of the titanium dioxide particles have particularly strong oxidizing power, many applications of titanium dioxide to paints, textiles, sick house syndrome alleviating agents and harmless treatment agents for industrial effluent and exhaust gas have been proposed, and a part thereof has been practically used.
Hitherto, the titanium dioxide fine particles that have been used for the photocatalyst are crystalline particles of anatase type or rutile type.
Since titanium dioxide of the anatase type or rutile type has a band gap of 3.2 eV (corresponding to a wavelength of 387.5 nm) or 3.0 eV (corresponding to a wavelength of 413.3 nm), respectively, only a UV light having a short wavelength of 387.5 nm or less or 413.3 nm or less is utilized, and visible light has not been utilized at all.
Accordingly, the crystalline titanium dioxide was difficult to function in the applications of interior paints, textiles and sick house syndrome alleviating agents that are used in a room where substantially no UV light exists, and the application of titanium dioxide has been restricted in actuality.
On the contrary, various kinds of titanium dioxide exhibiting catalytic activities by irradiating a visible light have been developed in recent years in order to efficiently utilize sunlight and artificial light.
For example, Japanese Patent No. 3,252,136 discloses titanium dioxide that can exhibit a catalytic activity induced by irradiating a light in a visible region and a method for producing the same, wherein the crystal lattice structure of titanium dioxide of the anatase type is made to be deficient in oxygen by a hydrogen plasma treatment or rare earth element plasma treatment under a reduced pressure, by rare earth element ion injection, or by heating at a high temperature in vacuum.
The titanium dioxide photocatalyst has been reported to respond to visible light by doping nitrogen in titanium dioxide (see “Nikkei Mechanical 2001. 10”, No. 565, pp.36–45). For example, Japanese Patent Application Laid-Open (JP-A) Nos. 2001-190953, 2001-207082 and 2001-72419 have disclosed that the titanium dioxide crystal involving oxygen in the crystal exhibits a catalytic activity by irradiating a visible light.
Titanium dioxide doped with nitrogen has a structure comprising doped nitrogen invaded in lattice interstices, or a structure in which oxygen sites in the lattice are substituted with nitrogen.
Such titanium dioxide doped with nitrogen may be synthesized by hydrolyzing titanium chloride in an aqueous ammonia solution, or by heating titanium dioxide in an ammonia gas.
The titanium dioxide photocatalyst is supposed to be activated by a visible light due to oxygen deficient defects or Ti—N bonds in the oxygen deficient or nitrogen doped titanium dioxide.